Molecules capable of mediating charge transport over several nanometers with minimal decay in conductance have fundamental and technological implications. Polymethine cyanine dyes are fascinating molecular wires because up to a critical length, they have no bond-length alternation (BLA) and their electronic structure resembles a one-dimensional free-electron gas. Beyond this threshold, they undergo a symmetry-breaking Peierls transition, which increases the HOMO-LUMO gap. We have investigated cationic cyanines with central polymethine chains of 5-13 carbon atoms (Cy3⁺-Cy11⁺). The absorption spectra and crystal structures show that symmetry breaking is sensitive to the polarity of the medium and the size of the counterion. X-ray crystallography reveals that Cy9·PF₆ and Cy11·B(C₆F₅)₄ are Peierls distorted, with high BLA at one end of the π-system, away from the partially delocalized positive charge. This pattern of BLA distribution resembles that of solitons in polyacetylene. The single-molecule conductance is essentially independent of molecular length for the polymethine salts of Cy3⁺-Cy11⁺ with the large B(C₆F₅)₄^- counterion, but with the PF₆^- counterion, the conductance decreases for the longer molecules, Cy7⁺-Cy11⁺, because this smaller anion polarizes the π-system, inducing a symmetry-breaking transition. At higher bias (0.9 V), the conductance of the shorter chains, Cy3⁺-Cy7⁺, increases with length (negative attenuation factor, β = -1.6 nm^-1), but the conductance still drops in Cy9⁺ and Cy11⁺ with the small polarizing PF₆^- counteranion.

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